It is quite common to use fully differential amplifiers in the analog front end of signal buffers or filters such as those in audio applications, where high dynamic range is desired. With a higher dynamic range, the signal-to-noise ratio is increased. FIG. 1 shows a conventional fully differential amplifier in its normal operation mode, while FIG. 2 shows the same differential amplifier undergoing an initial calibration process. It will be appreciated by those skilled in the art that calibration is needed for these amplifiers due to the differential offset voltage from the differential pair transistors, which reduces the effective dynamic range of the circuit. The goal for calibration is therefore to determine the current needed to effectuate offset cancellation.
Reference is to FIG. 2. During the calibration cycles, the amplifier is converted into a single-ended comparator by having both inputs of the amplifiers grounded to analog ground ("AGND") and using switches to make the amplifier open-loop and single-ended. The positive output ("OUTP") will sit at the positive or negative supply rail, while a current is injected through Current-In. As more and more Current-In is applied to the amplifier, the voltage level of the output OUTP will flip sooner or later to the other supply rail and therefore cross AGND. Note that for analog applications, the AGND is typically half-way between the positive and negative supply rails. At this time, the current needed to cancel the differential offset voltage is determined. After calibration, the normal operation resumes as in the configuration of FIG. 1 with the now-determined Current-In consistently applied to cancel the differential offset of the amplifier.
Referring to FIG. 3, where a schematic diagram of the conventional differential amplifier of FIG. 2 is shown. Again, during calibration cycles, i.e. CAL is active, with both inputs INP and INM grounded to AGND, Current-In is supplying current to either branch pa or pb, depending upon whether Q or QB is active. (Note: Q and QB are complementary signals.) If Q is active, current is injected to the branch pa to ramp down the output voltage OUTP until OUTP's level crosses AGND. When AGND is crossed by the output OUTP, the current applied to branch pa is thus the requisite current to cancel the offset voltage of the differential pair.
If, however, the maximum current is reached and the output OUTP never crossed AGND, it indicates that the offset voltage has another polarity, e.g. offset &lt;0. In this case, QB turns high and begins injecting current into branch pb to cause the voltage level of OUTP to ramp up and to cross AGND. This will determine the current, albeit in another direction, necessary to cancel the offset voltage of the input differential pair.
As described, the conventional approach to offset cancellation has been to reduce the differential offset by injecting a current, I, in one of the two branches of the differential input pair. For fully differential amplifiers, this approach has resulted in a common-mode offset in that the differential offset is now, effectively, shifted by the injected current to the common-mode circuitry of the differential amplifier. Referring to FIG. 3, the total current out of branches pa and pb is now greater than the current out of the current source CS due to the injected current from Current-In. To compensate for the difference, the common-mode circuitry (not shown) needs to generate an offset voltage, which will then adversely affect the dynamic range of the amplifier.
Therefore, it is desirable to have a differential amplifier with offset cancellation of the differential signal without affecting the offset of the common-mode circuitry.
It is also desirable to achieve offset cancellation without increasing complexity of the circuit.